Two dimensional chiral networks emerging from the aryl-F...H hydrogen-bond-driven self-assembly of partially fluorinated rigid molecular structures

Article abstract of DOI:10.1021/ja801925q

Self-assembly of the partially fluorinated rigid molecules physisorbed at solution/graphite interface has been investigated by scanning tunneling microscopy. Upon adsorption, both the branched star-shaped compound 1 and the angulate rod 2 compromising diacetylene and acetylene interlinked benzene and pentafluorobezene formed two-dimensional chiral porous networks. The spontaneous formation of these architectures is likely attributed to the two effects: the compensation of the dipole moments of the branches and the formation of Ar-H...F hydrogen bonds. These results demonstrate that the immobilization of molecules at the liquid/solid interface can be driven by these weak intermolecular interactions instead of van der Waals interactions between alkyl chains and substrate. Copyright

Full text of DOI:10.1021/ja801925q

Published on Web 07/24/2008
Two Dimensional Chiral Networks Emerging from the Aryl-F · · ·H
Hydrogen-Bond-Driven Self-Assembly of Partially Fluorinated Rigid Molecular
Structures
Zhongcheng Mu,† Lijin Shu,‡ Harald Fuchs,† Marcel Mayor,*,‡,§ and Lifeng Chi*,†
Physikalisches Institut, Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster, Wilhelm-Klemm-Str. 10, 48149 Mu¨nster,
Germany, Institute for Nanotechnology, Forschungszentrum Karlsruhe GmbH, Postfach 3640, 76021 Karlsruhe,
Germany, and Department of Chemistry, UniVersity of Basel, St. Johannsring 19, 4056 Basel, Switzerland
Received March 14, 2008; E-mail: marcel.mayor@unibas.ch; chi@uni-muenster.de
Self-assembly of organic molecules at the solid/liquid interface
has attracted particular attention to provide patterned surfaces,
spatially defined arrangements of molecules, laterally controlled
supramolecular architectures, and as case study of two-dimensional
crystal engineering.1 While the surface deposition is entropically
driven, the molecular arrangement on the surface is controlled by
intermolecular interactions like hydrogen bonding and/or van der
Waals interactions as well as molecule/substrate interactions. Of
particular interest are self-assembled systems yielding in porous
networks, providing a well-defined two-dimensional (2D) pattern
as template for functional guests, such as quantum dots.2 So far,
2D porous networks have been assembled by directional intermo-
lecular forces like hydrogen bonding,3 metal-ligand interactions,4
and van der Waals interactions of interdigitating alkyl chains.5
Perfluorobenzene subunits have been used as rather electron-
poor aromatic building blocks to tailor inter- and intramolecular
properties. Stacking with electron-rich aromatic systems as a
supramolecular motif has been investigated in detail6 and has been
applied successfully in supermolecules,7 crystal engineering,8
topochemistry,9 material design,10 and even in biomolecules.11 At
the single molecule level, perfluorobenzene subunits unbalanced
the electron distribution on a molecular rod, providing rectification
properties.12
Figure 1. (a) Chemical structure of 1. (b) Large-scale STM image of 1
(50.0 nm × 50.0 nm, Vbias ) 0.5 V, Iset ) 0.60 nA). (c and d) High-resolution
STM image of 1 (18.0 nm × 18.0 nm, Vbias ) -0.5 V, Iset ) 1.43 nA). (d)
A domian displaying a mirrored arrangement compared with (c). In both
cases, six molecules of 1 are overlaid to illustrate both the proposed packing
model and the resulting chiral cavities.
At the solid/liquid interface, alkylated molecular rods comprising
a perfluorobenzene subunit yielded parallel stripes as a perfectly
ordered large area surface pattern.13 Of particular interest is the
unbalanced electron distribution in these rods providing a dipole
moment which might support or even drive the molecule’s
arrangement at the solid/liquid interface.
Here we present new rigid-rod-type structures forming well-
defined 2D porous networks comprising chiral cavities. Both
structures, the branched star-shaped compound 1 and the angulate
rod 2 (Figures 1a and 2a), have a central core consisting of acetylene
interlinked aromatic subunits, and their branches are terminally
functionalized with pentafluorobenzenes connected via diacetylenes.
As displayed in Scheme 1, in a three-step synthesis, both
structures 1 and 2 were assembled. Starting from commercially
available 1,3,5-triiodobenzene (3) or 1,3-diiodobenzene (6), the
iodines are substituted by monoprotected para-diethynylbenzene
in a Sonogashira coupling reaction. After deprotection, the free
acetylenes are capped with bromoethynylpentafluorobenzene in a
palladium-catalyzed version of the Cadiot-Chodkiewicz reaction
to provide both target structures 1 and 2 as brown solids in yields
of 52 and 55%, respectively. Detailed synthetic protocols are
provided in the Supporting Information (SI-1).
Figure 2. (a) Chemical structure of 2. (b) High-resolution STM image of
2 (12.0 nm × 12.0 nm, Vbias ) 0.5 V, Iset ) 0.5 nA). Four molecules of 2
are overlaid to illustrate the proposed packing model.
The structures 1-7 have been fully characterized by 1H and 13
C
NMR spectroscopy, mass spectrometry, and elemental analysis.
However, the combination of very limited solubility of the
compounds with low signal intensity due to extensive F coupling
did not allow observing the 13C signals of the fluorinated carbons.
The self-assembly properties of 1 and 2 have been studied at the
solid/liquid interface by depositing a droplet of a nearly saturated
solution of 1 or 2 in phenyloctane on a highly oriented pyrolytic
graphite (HOPG) surface. Subsequent investigation of the surface by
STM displayed the formation of a homogeneous and ordered large
area honeycomb structure in the case of 1 (Figure 1b). The bright areas
correspond to the π-conjugated molecular backbones due to high
electronic densities, while the dark areas are voids in the network.
The high-resolution STM image (Figure 1c) even allows identifying
† Westfa¨hlische-Wilhelms-Universita¨t Mu¨nster.
‡ Forschungszentrum Karlsruhe GmbH.
§ University of Basel.
9
10840 J. AM. CHEM. SOC. 2008, 130, 10840–10841
10.1021/ja801925q CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Synthesis of the Molecular Star 1 and the Molecular
Angle 2a
-5.5 ( 2° between the unit cell vector a and the main symmetry
axis of HOPG was observed as shown in SI (Figure S3).
In conclusion, the self-assembly of two molecular structures 1 and
2 comprising diacetylene and acetylene interlinked benzene and
pentafluorobenzene branches at the solid/liquid interface resulting in
two-dimensional chiral porous networks is described. The spontaneous
formation of these stable large area surface architectures is assumingly
driven by the compensation of the dipole moments of the branches
and the formation of aryl-H· · ·F hydrogen bonds. The absence of
alkyl chains, which are known to direct the self-assembly of rigid
molecular cores,5 is noteworthy. Currently, we are further function-
alizing these synthons to profit from their large area periodicity.
a Conditions: (a) HC2(C6H4)C2Si(C3H7)3, Pd(PPh3)4, N(Et)3, CuI, 40 °C,
20 h, 4: 77%, 7: 94%; (b) TBAF, wet THF, rt, 5: 71%, 8: 80%; (c)
Pd2(dba)3 ·CHCl3, (i-Pr)2EtN, CuI, toluene, rt, 3 h, 1: 52%, 2: 55%.
the branches of individual star-shaped 1. The parameters of a unit cell
have been determined to be a ) b ) 3.3 ( 0.2 nm, R ) 60 ( 2°. The
length of the individual bright stripe corresponds with 2.14 nm to the
length of the molecular branches (2.16 nm by MM2 calculation). Two
adjacent branches belonging to two different molecules are antiparallel
to each other, maximizing their overlap. The terminal pentafluoroben-
zene units point to the meta-di(phenylethynyl)benzene center of the
neighboring molecule allowing the formation of three aryl-H· · ·F
hydrogen bonds (SI, Figure S1). Thus, in the resulting porous 2D
network, two neighboring molecules are stabilized by both antiparallel
arrangement of the branches comprising a dipole moment and the
formation of six aryl-H· · ·F hydrogen bonds, which have already been
reported for monolayers consisting of self- assembled fluorinated
phthalocyanines.14 The crucial role of this intermolecular stabilizing
interaction was further emphasized by control experiments with the
structurally related star-shaped molecule lacking the F atoms (see 11
in Supprorting Information SI-1), which failed to form self-assembled
monolayers under similar conditions.
Acknowledgment. The authors acknowledge the support with
a postdoctoral position for L. Shu by the Center for Functional
Nanostructures (CFN) of the Deutsche Forschungsgemeinschaft
(DFG) within project C3.
Supporting Information Available: Synthetic protocols and ana-
lytical data of all mentioned molecules, the additional STM images
and the molecular packing model are available This material is available
free of charge via the Internet at http://pubs.acs.org.
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